In the state of the art very diverse methods of preparing the end have been described.
In UK Patent No. 1480399 in the name of Stahlecker, the thread is prepared by picking up the thread end by means of a rotating gripper and subsequently untwisting the thread by rotating the gripper in the opposite direction to the direction of twisting. This thread preparation method has its drawbacks since from time to time the thread end may have a different and indeterminate residual twist. In fact the free end tends to untwist spontaneously, at least in part, and this untwisting spreads along the thread. It is not possible, therefore, to predict beforehand how many turns the gripper must make in order to render the fibers essentially parallel. If these turns are too few, the fibers of the thread end are again twisted; if, however, the turns are too many, the fibers are twisted in the opposite direction. This gripper also pulls the thread damaging its fibers.
In U.S. Pat. No. 3,925,975, in the name of Stahlecker, the thread end is untwisted by pairs of rubbing pads. This type of preparation is also affected by the degree of variability of the twist condition of the thread end.
Moreover, in the state of the art, in spinning frames currently in production, rapidly rotating milling cutter devices are used, the thread being brought towards them, stretched out by the effect of an air suction flow. Milling of the thread produces a tapered thread end with the fibers parallelised by the stress induced by the projections of the milling cutter.
The drawbacks of this technical solution are that the end thus prepared is not of a constant length, the milling cutter wears and becomes blocked due to coils of yarn winding round it and the thread end is insufficiently tapered. The fibers are shortened by the action of the milling cutter.
An alternative technical solution applied industrially comprises a preparation device in which the milling cutter is replaced by a rotating abrasive disc or other abrasive bodies, as in German Patents Nos. 2350842 and 2350843 in the name of Stahlecker. Other state of the art methods of preparing the end consist in cutting the thread to the required size and untwisting and fraying out the end with jets of air directed at right angles, parallel or inclined in relation to the axis of the thread, eliminating the twists and removing the short fibers, as in German Patent No. 2361787 in the name of Stahlecker.
This type of preparation produces an insufficiently tapered end, giving rise to a strong but thickened join which gives an irregular and variable result.